Subsea equipment is typically hydraulically actuated. To effect actuation, deepwater accumulators often provide a supply of pressurized working fluid that helps control and operate the subsea equipment. This pressurized working fluid (e.g., hydraulic fluid) may be used to operate underwater process valves and connectors, and/or to manage fluid power and electrical power on subsea drilling BOP stacks, subsea production Christmas trees, workover and control systems (WOCS), and subsea chemical injection systems, to name but a few possibilities.
Accumulators are typically divided vessels with a gas section and a hydraulic fluid section of adjustable volumes. Accumulators operate on a common principle: The gas section is precharged with a gas at a pressure equal to or slightly below the anticipated minimum pressure required to operate the subsea equipment. As working fluid is added to the accumulator in the separate hydraulic fluid section, the volume of that section increases. In turn, the volume of the gas section is reduced, thus increasing the pressure of the gas and the hydraulic fluid. The hydraulic fluid introduced into the accumulator is therefore stored at a pressure at least as high as the precharge pressure and is available for doing hydraulic work.
The precharge gas can be said to act as a spring that is compressed when the gas section is at its lowest volume/greatest pressure and released when the gas section is at its greatest volume/lowest pressure. Accumulators are typically precharged in the absence of hydrostatic pressure, and the precharge pressure is limited by the pressure containment and structural design limits of the accumulator vessel under surface (ambient) conditions. Yet, the efficiency of conventional accumulators decreases in deeper waters because hydrostatic pressure and lower temperatures can cause the non ideal gas to compress, leaving a progressively smaller amount of useable volume of hydraulic fluid to power the subsea equipment's functions. The gas section must consequently be designed such that the gas still provides enough power to operate the subsea equipment under hydrostatic pressure even as the hydraulic fluid approaches discharge and the gas section is at its greatest volume/lowest pressure.
For example, BOP mounted accumulators at the surface typically provide 3000 psi of working fluid maximum pressure. At a depth 1000 feet below the sea surface, the ambient pressure (i.e., hydrostatic pressure) is approximately 465 psi. Thus, to provide a 3000 psi of differential pressure at a depth of 1000 ft, the accumulator has a precharge of 3465 psi, which is 3000 psi plus 465 psi. At a depth of slightly over 4000 ft., the ambient pressure is almost 2000 psi, making the effective precharge 5000 psi, which is 3000 psi plus 2000 psi. This would mean that the surface precharge would equal the working pressure of the accumulator, and any fluid introduced for storage or temperature increase after precharge may cause the pressure to exceed the working pressure and significantly degrade performance of the accumulator.
At progressively greater hydrostatic operating pressures, the accumulator thus has greater pressure containment requirements than at non-operational (no ambient hydrostatic pressure) conditions. The inefficiency of precharging accumulators under non-operational conditions thus requires large aggregate accumulator volumes that increase the size and weight of the subsea equipment. With rig operators increasingly putting a premium on minimizing size and weight of the drilling equipment to reduce drilling costs, the size and weight of all drilling equipment must be optimized. With deeper drilling depths, more and larger accumulators are required, increasing not only the size and weight of the subsea equipment, but also the rig equipment used for transport and handling of the subsea equipment.
Accumulators may be included, for example, as part of a subsea BOP stack assembly assembled onto a subsea wellhead. Fluid pressure, supplied by the accumulators can be used to operate the rams of the BOP. The BOP assembly may include a frame, BOPs, and accumulators to provide hydraulic fluid pressure for actuating the rams. The space available for other BOP package components such as remote operated vehicle (ROV) panels and mounted controls equipment is being reduced due to the increasing number and size of the accumulators required to for operation in deeper water depths. When a function of a subsea control system is activated, most of the high pressure fluid stored in the subsea or surface accumulators is used to move the function to the close position or the shear rams onto the pipe. It is desirable to minimize use of the high pressure stored fluid for movement of the function, but use it to actually perform the work to create a seal or shear the pipe as this will reduce the amount of accumulators that have to be installed on surface and on the BOP stack. Consequently, techniques for reducing the fluid pressure and high pressure fluid volume requirements of subsea equipment, and correspondingly reducing the need to increase surface and subsea accumulator capacity are desirable.